The invention relates to electron beam coating systems. More particularly the invention relates to an improved system for cooling the top plate of a multi-pocket electron beam gun.
Electron beam guns have frequently been employed to evaporate material in a high vacuum environment for the purpose of ultimately depositing the material on a specific article or location. Such deposition systems typically include an electron emitting source, a crucible for holding the evaporant, and means for directing the electron beam onto the surface of the evaporant. Examples of such systems are described in U.S. Pat. Nos. 3,710,072, 3,235,647 and 3,420,977.
One important development in the electron beam coating art has been the development of rotatable tables containing multiple pockets for holding evaporants. A multi-pocket electron beam gun may be loaded at a single time to deliver different evaporant materials. The gun is set up to deliver different evaporant materials by simply rotating the table in which the multiple pockets are contained. The multi-pocket electron beam gun design also provides increased capacity over the conventional single pocket design.
A necessary element of the multi-pocket electron beam gun is a top plate cover for the table, which allows only one pocket at a time to be exposed to the sweeping electron beam. Usually the top plate has a hole or cut-out section dimensioned to allow exposure of a single pocket. The top plate edge adjacent to the cut-out section, the "inward edge", is inherently susceptible to excessive heating because of it proximal location to the electron beam target.
A common problem with the multi-pocket electron beam gun is that evaporant collects on the inward edge and later re-evaporates contaminating the item to be coated.
In a prior device, inventors have attempted to minimize the problem by causing coolant to flow through a straight channel which runs longitudinally through the top plate of the electron beam gun. See FIG. 2. The straight cooling channel, however, inadequately cools portions of the top plate distal from the channel.
It has been discovered that a primary cause of the contamination problem is that the top plate edge adjacent to the exposed evaporant pocket inherently becomes coated with evaporant because of its close proximity to the evaporant pocket. In subsequent use the coated edge of the top plate becomes hot from secondary electron bombardment and from heat emanating from the evaporant reservoir, causing the evaporant to re-evaporate finally condensing on and contaminating the item which is intended to be coated exclusively with material from the exposed pocket.
Another problem related to excessive heating of the top plate edge is that the heat causes the condensed evaporant to fuse to the plate, making it difficult to clean.
Another problem is that excessive heating of the top plate edge causes it to warp.